Isolette/Compur minimum aperture

[Kevin Brown]

Has anyone experimented with/measured the smallest apertures on the Compur Rapid used on many Isolettes (and others); the markings only go to f/22, but the iris closes down way further past that point. I haven't taken off the front element to actually measure, but just eye-balling, it looks like it may go down as far as f/45!

Though most avoid apertures smaller than 22 on medium format due to diffraction limitations, unless you are only doing enlarger, chemical development prints, as many have shown in various online articles, the effects of diffraction can be for all practical purposes completely eliminated by careful application of sharpening.

The considerable increase in depth of field between f/22 and f/45 can be of great value, especially when using a scale-focus medium format camera.

 

[Dwig]

...the effects of diffraction can be for all practical purposes completely eliminated by careful application of sharpening. ...

Not really. Digital sharpening can't magically create detail that is not present in the original image. It can give an impression of "sharpness" but can't create new missing detail.

 

[Kevin Brown]

No, but small aperture-induced diffraction doesn't reduce detail; it only creates a spread of light around the details created by the lens, reducing the visibility of the 'detail'; rather the way a fog filter does, but on a micro level. In other words, diffraction doesn't make an image formed by the lens less 'in focus', rather it adds a 'fuzz' around the details the lens is creating; digital sharpening undoes this very neatly, making the details more clearly visible.

As shown here:

https://jonrista.com/2013/03/24/the-diffraction-myth/

 

[farlymac]

I have a Polaroid 110B which has the facility for using an f90 aperture via the lens cap, and I don't recall much in the way of diffraction issues when I used it. Maybe because the aperture is created in front of the lens, instead of in the middle. BTW, it made everything from 22 inches to Infinity in focus. Man, I miss Polaroid roll film.

As for the Isolette aperture settings, I rarely ever went beyond f16, so no experimenting with the minimum ever occurred. But now you have me thinking.

PF

 

[Kevin Brown]

Keep in mind, visibility of diffraction is entirely dependent on the physical size of the opening in relation to the film format size - so with that big 3 1/4 x 4 1/4 Polaroid neg, f/90 is about equivalent to f/22 or f/32 on medium format.